The Medical Letter on Drugs and Therapeutics
Drugs for Hypothyroidism
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Primary hypothyroidism is usually the result of Hashimoto's thyroiditis, thyroidectomy, or radioactive iodine therapy. Treatment of hypothyroidism with replacement doses of thyroid hormone is usually lifelong. Levothyroxine is the drug of choice.

LEVOTHYROXINE (LT4) — Pharmacokinetics – In healthy fasting adults, 70-80% of an oral dose of LT4 is absorbed in the jejunum and ileum. LT4 has a half-life of about 7 days, permitting once-daily dosing and maintenance of a steady state despite occasional missed doses. Steady state is reached after about 6 weeks of treatment. Maintaining a steady-state level in the blood ensures sufficient tissue concentrations for peripheral deiodination of biologically inactive T4 into active triiodothyronine (T3).

Dosage – LT4 has a narrow therapeutic index. The usual replacement dose is about 1.6-1.8 mcg/kg/day. Patients with thyroid cancer taking LT4 to suppress ... more      

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